reactor-advanced

Reactor.Advanced Win2D canvases — choosing Win2DCanvas, Win2DAnimatedCanvas, or Win2DVirtualCanvas; using UseDrawState, UseCanvasResources, UseDrawCommand; and following threading/device-loss rules.

Reactor Advanced

Use this skill when adding immediate-mode Win2D drawing through Microsoft.UI.Reactor.Advanced. The full guide is docs/guide/win2d-canvas.md; the performance sample is samples/apps/particle-storm/.

Pick the right canvas

Workload	Use	Why
Static or data-driven drawing that changes after state updates	`Win2DCanvas.Of(onDraw, redrawKey)`	Manual invalidation; pass a changing `RedrawKey` for every value the draw callback reads.
Game loop, physics, particles, visualizers, steady FPS	`Win2DAnimatedCanvas.Of(onUpdate, onDraw, drawState, isPaused)`	Win2D owns the tick and calls update/draw on the game thread.
Huge scrollable or tiled surfaces	`Win2DVirtualCanvas.Of(onRegionDraw, contentSize)`	Draws only invalidated visible regions; update tiles by changing `InvalidateRegions`.

Threading cheat sheet

Callback	Thread	Rule
`Win2DCanvas.OnDraw`	UI thread	Safe to read Reactor state captured by render.
`Win2DAnimatedCanvas.OnUpdate` / `.OnDraw`	Win2D game thread	Do not touch WinUI controls. Use thread-safe data or `UseState(threadSafe: true)` for UI handoff.
`Win2DVirtualCanvas.OnRegionDraw`	UI thread	Keep work bounded to the invalidated region.
`UseCanvasResources` factory	Worker/game thread	Create device resources and return a disposable/recreatable object.

Treat Ref.Current from UseDrawState like a volatile field. Re-read it and make the referenced object safe for any cross-thread mutation. Debug builds add a sentinel that appends docs/guide/win2d-canvas.md#threading to likely WinUI thread-affinity exceptions from animated callbacks.

Reference graph model

Element refs are reactive cells. .Ref(cell) writes the mounted FrameworkElement into the cell and writes null on unmount. Reference properties (TeachingTip.Target, .LabeledBy, .DescribedBy, .FlowsTo, .FlowsFrom, .XYFocus*, and custom descriptor .Reference / .ReferenceList) subscribe to those cells instead of sampling .Current.

Resolution is one-way and push-based: cells enqueue dirty reference edges during commit, then Reactor flushes the dirty set after the tree is stable. Cycles are allowed because each property slot is a one-way edge. List references preserve author declaration order and omit unresolved targets. Advanced authors should use descriptor.Reference for regular controls and binding.Reference only as the bridge for hand-coded handlers.

Hooks and recipes

var state = ctx.UseDrawState(() => new ParticleField(20_000));

var sprite = ctx.UseCanvasResources<CanvasBitmap>(async device =>
    await CanvasBitmap.LoadAsync(device, "Assets/particle.png"));

var draw = ctx.UseDrawCommand(model, static (session, args, m) =>
    m.Render(session), deps: [model.Version]);

UseCanvasResources is the device-loss recipe: allocate from the supplied CanvasDevice, draw only when the returned ref is non-null, and let the hook dispose/recreate after CanvasDevice.DeviceLost.

Performance proof

The Particle Storm sample (samples/apps/particle-storm/) is the canonical pattern: pure Reactor chrome controls parameters; Win2DAnimatedCanvas renders the hot particle path; UseDrawState holds the particle buffers; UseCanvasResources owns sprites and other device resources.